Automatic gain compensating means



May 12, 1942. F. J. FALTICO AUTOMATIC GAIN COMPENSATING MEANS Filed Ma a, 194 s Sheets-Sheet 1 FRANK J fALTlco May 12, 1942.

I F. J. FALTICO 2,282,649

AUTOMATIC GAIN COMPENSAT ING MEANS Filed May 8, 1940 3 Sheets-Sheer. 2

FRANK J fitLT/CO 3 G M W aftomma,

May 12, 1942. J -ncc Y 2,282,649

AUTOMATIC ,GAIN COMPENSATING mmus Filed May 8, 1940 3 Sheets-Sheet s fgi0 Patented May 12, 1942 UNlTED STATES PATENT OFFICE 2,282,649 5 AUTOMATIC GATN CQMPENSATING MEANS Frank J. Faltico, Minneapolis Application May 8, 1940,.Serial No. 333,998, soiaim'sg (c1.179-'-17 1) This invention presents a novel and extremely efiective means for controlling the amplification gain and frequency characteristics, of an amplifier using vacuum tubes.

The primary object of the invention is to provide means to automatically, continuously, and mstantaneouslywontrol the output level of an amplifier, and providing the effects of volume suppression and volume expansion to thereby maintain substantially uniform output from either loud or weak input signals.

The invention is primarly designed for use in such speech or audio amplifiers as those'used in hearing aid units, wherein it is extremely desir able to provide means for instantaneously 1im+ iting the output from loud signal inputs and thus prevent pain to the user. At the same time, it is likewise desirable to provide such limiting or volume suppression action without the heretofore concomitant suppression or reduction in the weak signal response of the amplifier. Thus it has been found that in an amplifier employing my invention the loud signal inputs will be held to any predetermined comfortable output level but the weak signal response will be increased to such a point that they will not be masked by such loud signals. This is accomplished by shifting the slope of the dynamic characteristic curves of the amplifier tube, or tubes, automatically in practically the same manner as the characteristic curve of the ordinary ear is shifted to accommodate the difference in input signal levels; and thus the hard of hearing person, in whose ears due to their impairment this control is absent, is provided with all the benefits of the nor- 5 mal hearing characteristics.

The invention, however, is not limited in its application to hearing aid work but may be used wherever vacuum tube amplifiers are arranged in cascade or series to amplify at either radio or high frequencies, intermediate frequencies, or audio frequencies. For example, my invention may be applied to the amplifiers used in both radio and television transmission and reception, motion picture and phonograph recording and reproduction, and the amplifiers used in sound detectors, medical equipment, and in all similar fields of work. i

The invention has been found also to have the characteristic of permitting any band offrequencies to be passed and amplified, and it is thus possible to control the band width and cut off any undesired frequencies in the outputof the amplifier. The'device thus admirably meets the requirements of communication work such as radio transmission, and the wire transmission of voice frequencies in telephone circuits.

A further object of the invention is to provide an'amplifier circuit in which one or more tubes is caused to have both aregenerating and degenerating action, with the relative proportions of these actions readily adjusted, to the point that the regeneration will greatly increase the amplification without at any time approaching the point of oscillation or instability due to the correlative degenerating action. A further object is to provide a circuit of this kind in which the vacuum tube elements are so connected that signal controlled variation in the tube transconductance is had with a resulting instantaneous volume control on signals of great intensity. Still a further object is to provide, in an amplifier circuit, an instantaneously and automatically operating trigger or phase inversion means by. whichloud signal suppression or signal ex- .pansion may be had without the heretofore necessary time delay or rectifier circuits. Still a further objectis to provide a circuit of the foregoing characteristics in which means for control of the tone or frequency characteristics may be readily had. 4

These andother more detailed and specific objects will be. disclosed in the course of the following specification, reference being had to the accompanying drawings, in which:

Fig, 1 is a diagrammatic representation of a vacuum tube amplifier circuit embodying what may be termed the fundamental version of my invention. 7 r

Fig.2 is a diagrammatic showing of the dy namic characteristic curve of a vacuum tube amplifier, and illustrating the manner in which the slope of such curve is varied in accordance with my invention.

Fig. 3 is a diagram of another version of my invention embodying in this case three amplifier stages and still further illustrating a novel and effective tone control means.

. Fig. 4 is a circuit diagram of a single vacuum tube amplifier having provision for degenerating and regenerating action under control of signal induced currents and following stage. This view also illustrates a further modification-of the tone or frequency control device.

.Fig. 5 is a view similar to Fig. 4, but employing a difierent type of multi-grid vacuum tube. Fig. 6is a-circuit somewhat similar to that'of Fig. 1, showing a filter arranged in the feedback circuit as a control for the band width or frequency amplified by the regenerative circuit.

Fig. '7 is another somewhat similar circuit but illustrating another application of the degenerative and regenerative circuit, and showing still further a modified connection of the second amplifier tube as a space charge amplifier.

Fig. 8 is a view similar to Figs. 4 and 5 but illustrating the use of a common control grid, and the proper connection of the regenerating and degenerating resistor thereto.

Fig. 9 is a diagrammatic representation of the fundamental circuit embodying instantaneous operating trigger amplifier or phase inverter as a volume suppression means.

Fig. 10 is a circuit showing still another application of the trigger amplifier in connection with two cascaded stages of amplification.

Fig. 11 is a circuit showing another modification of the trigger amplifier, in this case connected directly to the signal input means.

Fig. 12 is a circuit similar to that of Fig. 3, embodying the regenerating and degenerating action thereof and with the addition thereto of the phase inversion means.

Fig. 13 is a partial circuit diagram of two cascade stages of amplification showing a push-pull or rectifier type of trigger circuit in connection therewith.

Referring now more particularly and by reference characters to the drawings, the circuit of Fig. 1 is that of two amplifying stages embodying vacuum tubes T and T of the conventional pentode type. As shown, the first tube T receives its input signal from a microphone M, although, as a matter of course, the tube may follow any other source of signal input depending upon its use and the particular type of circuit in which the tube is employed. The input circuit of the tube T is conventional in all respects and the output from the plate of this tube is connected to the usual coupling condenser ID to the input circuit of the tube T The conventional grid or biasing resistances II and I2 are used in both grid circuits and arranged in the plate or output circuit of the tube T is a plate load resistance [3 and a coupling condenser 14 leads the signal output to any following amplifier or reproduction device, it being understood, of course, that this output circuit may take any form desired.

The tubes T and T being, as stated, of the pentode type, embody therein the screen grids designated at 15 and 16, which in accordance with conventional practice, must be provided with a direct current potential from the B supply or plate voltage supply represented at l1. As ordinarily connected these screen grids are provided with a direct current potential less than that on the plates of the tubes through a series resistance or bleeder and the screens are by passed directly to ground.

The plate of the tube T like that of the tube T is provided with a load resistance l8 and in accordance with my invention the screen grid l5 of tube T is connected directly to the plate through a resistance l9. A tap at on this resistance I9 is then connected back to the screen grid l6 of the tube T it being understood of course that this resistance l9 may be either an adjustable one, or that it may be composed of two resistance units with the tap 20 made at the junction thereof.

By the novel connection of the screen grid l5 directly to the plate of the tube T I secure "a. novel effect and operation which will now be described. It is well known in the art that the function of the screen grid is to hold up space charge conditions in the tube during positive halves of the control grid voltage swing and that the operating potential on this screen or space charge grid is therefore critical. The tube T is initially set to operate at a point at which distortion will occur when the input signal exceeds a predetermined level and such distortion, as is also well known, will introduce a shift in the direct current component of the signal which, flowing through load resistor l8, will cause an instantaneous and pronounced voltage drop therein by the i3 formula. This resistor I8, being, in accordance with any invention, common with the screen grid [5, will thus as this voltage drop occurs reduce the operating potential upon the screen grid lowering space charge conditions in the tube T and instantaneously reducing the tube transconductance and gain. There thus occurs an instantaneous reduction in gain upon signals of a predetermined maximum, which is determined by the constants of circuit components and choice of operating potentials.

Through the connections of screen grid 6 of tube T back to a point on the resistance I9 between the screen grid and plate T still a further novel and peculiar action is set up as will now be described. The screen grid of the tube '1 is degenerative to its control grid since said control grid is connected through coupling condenser ID to the plate of T At the same time this screen grid I6 is regenerative with respect to the screen grid of T and a part of the signal is fed back or regenerated from the screen of T to both the screen and plate of tube T in proportions determined by the ratio of the respective sections |9a and IQ!) of the resistance 19. That portion of this signal fed to the plate of T has a degenerating action since it is then impressed on the control grid of T while that part of the signal fed to the screen grid 16 has a regenerating action to increase the overall gain. There thus results an automatic volume expansion for weak signals which is stabilized by the degeneration to the plate of T to prevent oscillation on weak and regenerated signals. Oscillation on stronger signals is prevented by the automatic gain reduction caused by the con- 'nection between screen grid and plate, as described hereinbefore.

This combination of the effects of regeneration and degeneration has been found to provide all of the desirable characteristics of increased gain without any tendency of the amplifier to go into oscillation, the inherent stability in the circuit being provided by the degenerative action.

The effect of the foregoing action is best illustrated in Fig. 2, wherein is shown the diagrammatic representation of the dynamic characteristic curve or load line of avvacuum tube. This line indicated at L is shifted toward the position indicated at L by the effects of regeneration and is conversely shifted towards the direction indicated at L by the effects of degeneration. The input signal is indicated at I and the output signal at O and it follows therefore that the variation of the slope of the line L has a direct bearing and control on the intensity of the output signal relative to that of the input signal.

Referring now to the circuit of Fig. 3, the vacuum tubes T and 'I therein take substantially the same functions as those-described herebefore, the tube T amplifying the signal from the source M and transferring the signal through coupling of both tubes T and T are again supplied with,

direct current potential from the plate voltage supply l1, and tube T has the plate load resist ance l8. 7

Again in accordance with my invention, ,the screen grid of the tube T is connected to its plate through the resistance l9, tosecure automatic gain control on load signals, but the tap or junction point on this resistance, instead of being connected to the screen grid of the following tube, is in this case connected by a tap ZI to the plate load resistance of the tube T The vacuum tube '1 is then connected as the ordinary pentode by the use of a separate screen grid feeding resistance 22 by-passed directly to ground at 23. The connection from the plate loadresistance l3 will again feed back a varying voltage to the screen grid I5 and plate of the tube T in proportions varying in accordance with the ratio between the respective sections or portions l9a and I9?) of resistance IS. The hereinbefore described actions of voltage control regeneration and degeneration will thus take place controlling the gain of the amplifier in a desired manner and it is to be seen, therefore,the source of signal voltage for application to the tube T may be taken from any signal carrying portion of the tube T or possibly that of succeeding amplifiers. The resistance I3 may of course be either variable in order to permit adjustment of the signal voltage fed back through tap 2|, or may be composed of two properly proportioned separate resistances.

Referring now to the circuit of the tube T it is noted that the usual grid resistance is used at 24 and any usual output coupling may be employed. The screen grid 25 of this vacuum tube is, however, connected to the plate through a. resistance 26 in substantially the same manner as the corresponding circuit of T and it has been found that by connecting a combination of a bypass condenser and variable resistance or impedance between this screen 25 and ground that a very effective tone control may be provided. As here shown, the condenser 21 and variable resistance 28 are employed and the frequencies thus by-passed to ground will have less degeneration or gain reduction within the tube and are therefore amplified to a greater degree, giving control of the frequency characteristic of the amplifier. Adjustment of the resistance 28 of course varies the frequencies at which this effect takes place. In this connection, it may be noted that the return of the condenser 21 and resistance 28 may be made back to an output portion of the circuit carrying a varying signal voltage instead of to ground, and in this case regeneration of the frequencies takes place rather than degeneration, again providing control of the frequency response characteristics of the amplifier.

Referring to Fig. 4, a single vacuum tube amplifier is indicated at T as receiving its input signal from the signal device M, and with output connection through the condenser 13 to the succeeding amplifier or signal translating device.

The plate circuit of this-tube has the usual load resistance 14 connected to the direct current po.- tential source 15, and the suppressor grid .16 is connected through resistance 11 back to the plate circuit to control space charge and gain in the tube on strong signals, The screen grid 18 is used as a control grid, and is connected to signal source M with the usualjbiasing resistance '19. The usual. control" grid 88 is then connected through resistance8l directly tothe plate, and connection to this resistance or to any part thereof is made through the lead 82 back to a following amplifier stage in order to feed back a signal controlled. voltage. The space charge connection of the tube thus formed increases the gain-while the total value of the resistance'8l determines thenormal, maximum gain of the tube. The regenerative, degenerative and automatic volume control effects hereinbefore mentioned are also set up in order to suppress loud signals while maintaining the weak signal response at a high value.

A tone control circuit may also be embodied in an amplifier of this type, and as here shown a choke coil 83 is connected between the suppressor grid 18 and ground through a variable condenser 84. Control of this .condenser 84 will vary the effective impedance offered by the choke 83, and thereby will permit the variation in the frequency response of the amplifien A quite similar circuit is shown in Fig. 5,

wherein the vacuum tube T is of the multi-grid type embodying a control grid85 to which the signal is fed from input device M, and which has the usual bias resistance 86. The plate circuit is conventional, and the screen grid 87 is connected throughresistance 88 to the plate circuit and by-passed by condenser 89 in the usual manner.

The second control grid 90 is in this case connected through resistance 9| directly to the. plate and the lead 92 carrying forward the signal controlled voltage as connected to this resistance 9|.

In operation the same type of automatic volume control regenerative and degenerative action is set up, in this case between the second control grid and plate, and the effects of loud signal suppression and weak signal expansion are again obtained.

Referring now to Fig. 6, there is represented therein a circuit similar to that of Fig. 1 embodying two vacuum tube amplifiers T and T and having the signal input source M, coupling condenser l0, grid resistances H and I2, and output circuit elements l3 and I4. The screen grids l5 and N5 of the tubes T and T are, how-' ever, in thiscase both connected directly to their respective plates through resistances l9 and I9 and connection between these resistances may, as shown, be made through a frequency discriminating unit 29. The plates of the tubes are connected to the.plat e-voltage supply I! through the plate load resistances l3 and I8, and the the source of the varying or control signal -voltage is taken from both the screen and plate of the tube T through resistance l9, but it is:

understood of course that it may be taken from either alone as may be desired. the frequency discriminating However, should unit 29 embody, therein a condenser, it is obviously necessary 'that the screen grid l6 have some direct current potential supplying connection with the rest of the circuit.

The connection of both screen g'rids l5 and I6 directly to plates of the respective tubes'T and T through resistances l9 and I9 provides control over the operating potential onthese screen grids to reduce the space charge, transconductance and gain in'both-tubes when'the tubes enter distortion on strong'signals and set up voltage dropping conditions in the plate load resist'anceslB and I3. The tubes T and T operate out of phase, of course, and the distortion entered is on opposite sides of the wave or grid voltage swing thus reducing the effect of such distortion on the final resultant signal. The automatic gain control action across two tubes is thus of considerable advantage. I

In this circuit, with the plates and screen grids of both tubes T and 'I connected by the respective resistances I9 and IS, the voltage drop across the plate load resistance l8 and I3 affects the transconductance of the tubes and thus automatically and instantaneously controls the gain through the circuit. On signal inputs of small intensity the full mutual conductance and I gain of the tubes is realized, but on larger signal inputs the gain is automatically reduced as has been previously pointed out. The reduction in gain occurs in the first tube T only on the negative half of the grid swing, so that so long as the signal intensity is small the output signal is symmetrical with the input signal, but of greater amplitude. As gain reduction takes place, however, the output signal wave form becomes unsymmetrical, since no reduction takes place on the positive peaks. Second harmonic distortion thus entered by the first tube is cancelled out by the second tube, which, operating 180- degrees out of phase with the first and on substantially the same portion of the characteristic curve, has a reverse effect on the wave form. The third harmonic distortion is of no moment in audio work. In other words, on the side of the wave form on which the first tube has had the effect of volume control the second tube has no efiect whatever, aside from further amplification, and so a return to a symmetrical wave form is secured. It is, of course, apparent that more than two tubes may be similarly cascaded and the circuit elements so proportioned that the second harmonic distortion cancellation is complete.

- The frequency discriminating unit 29 may embody any combination of condenser, resistance or impedance elements and ashere shown includes the condenser 33 and variable resistance 3|. The purpose of this unit is of course to provide control of the frequencies which are fed back for regeneration and degeneration purposes and thereby to control the overall frequency response of the amplifier.

The circuit shown in Fig. 7 represents a further modification of the basic degenerating and regenerating means thus far described but represents a special high gain version thereof. In this circuit the vacuum tubes T and T are arranged in cascade to amplify the signal from the signal source M and deliver the same to the load through the output coupling devices l3 and 14, the usual interstage coupling condenser l0 and biasing resistances II and I2 being employed. It will be noted that the resistance I2 is variable to act as a volume control or gain control in accordance with the'usual practice. The screen grid I4 of the first tube T is connected through a variable or adjustable resistance32 to the plate of the tube and the usual plateload resistance 13 i s also provided. This resistance 32 is ade justed to give-the maximum voltage gain in this 'stage without instability or oscillation and the action as regards volume suppression regenera-' tion and degeneration is substantially the'same as heretofore described in detail; 1 The vacuum tube T however, is connected in an unus'ual manner in that the control grid 33 is used as a space charge grid connected through resistance 33a to the plate, and the screen grid I6 is then used as a control grid. The suppressor grid is connected back to the screen grid I5 of the tube T to thus supply the necessary regenerating -voltage to this screen and the degenerating voltage to the control grid of the second tube; This connection of the second tube increases the gain of the stage, and the use of the suppressor grid as the source of the control voltage for the preceding tube further illustrates the flexibility of the possible connections which may be made in accordance with my invention; Fig. 8 shows a-circuit quite similar to Figs. 4

and 5 hereinbefore described, but in this case a standard screen grid amplifier tube T is used with the control grid 93 connected to signal input device M and biased through resistance 94. The plate circuit is again conventional, and is coupled through condenser 95 to the following amplifier tube T In this case, however, the resistance to which the lead 96 carrying the signal controlled current is connected is represented at 91, connected between the control grid 93 and plate. In order, however, to remove the positive potential from the grid the resistance 91 is actually connected to the following amplifier side of the coupling condenser 95, as clearly shown. Otherwise the operation is identicalwith that of the circuits hereinbefore covered.

The structure and operations as thus far described have been found in actual tests to be efiective in the proper control of the output over a relatively wide range signal input intensity, but in addition thereto I have found it desirable in some instances to employ what I believe to be an entirely novel trigger degenerative circuit or phase inversion means which will now be described. 1

Referring to Fig. 9, T represents an amplifier tube for amplifying the signal from the input device M and delivering the amplified signal through the output coupling condenser 35. The usual grid resistance 36 is employed as well as the plate load resistance 31 which is connected to the source of plate voltage represented at 38. The screen grid circuit represented generally at 33 is shown as being substantially identical with that of Fig. l, but the amplifier circuit of itself may be any of those conventionally employed.

The trigger amplifier or phase inverter tube T is connected across the amplifier tube T and the output of tube T is fed in part by the coupling condenser 40 to the control grid of tube T A coupling condenser 4| then connects the output of the tube T back to the grid circuit of the amplifier tube as clearly shown. The plate of the tube T is of course provided with a direct current potential through load resistance 41a from the plate voltage supply 38. The control grid of the-tube T is biased through the resistance 42 and is additionally provided with a certain fixed bias voltage by means of a bias cell, battery'or other voltage supply as indicated at 43.

Under conditions of normal or predetermined signal output level in the tube T the amount of signal carried to the trigger amplifier tube T is not sufiicient to overcome the bias provided in the grid of that tube. The amount of this bias,'and therefore the maximum level of signal which will be transmitted from the tube T to the next tube, or other output circuit, may of course be readily varied by increasing or decreasing the amount of fixed bias on the control grid of the trigger amplifier tube. When there appears a signal voltage at the plate of the tube T greater than the bias voltage on the grid of the tube T the difference in the voltage is amplified by the tube T and is impressed through coupling condenser 4! back on the grid of the tube T This signal voltage, however, is out of phase with the input signal on the tube T thereby decreasing the effective signal voltage and automatically and instantaneously reducing thegain through the amplifier circuit. It will be noted that weak sig: nals or signals of less intensity than those which cause this out of phase operation will have no effect whatever on the gain of the amplifier circuit and the maximum weak signalresponse will be obtained at all times. 7

' 'It will be noted that the circuit just described, as a corollary of the regenerating and degenerating means therein provided, may also be connected across two tubes of the amplifier and provided with a regenerating voltage such as to cause a volume expansion on weak signals along with the aforesaid volume suppression on strong signals. Such variation in the connection of the trigger amplifier or phase inverter is considered to be Within the scope of my invention.

The circuit shown in Fig. 10 is similar'to that of Fig. 9, and embodies the two cascaded vacuum tube amplifiers T and T receiving signal from the input device M for amplification and delivery through the output coupling condenser 9B'to the succeeding amplifier or signal translating device. The input biasing resistance 99 is used; and interstage connection is made through'the'coupling condenser I90 with the usual plate load resistances HH and I02 employed for feeding the direct current potential from'source I03 "to'the tubes. The second amplifier tube '1 is in this case shown as provided with theusual variable input resistance 104 for volume control purposes, but any other conventional grid circuit may of course be employed. The trigger amplifier or phase inverter tube T has its control grid con nected through coupling condenser I05 to the plate or output of the first amplifier tube T and this control grid of the tube T is provided with'a variable grid resistance I06 and with a fixed bias I01. The platecircuit of the tube T has the load resistance I08 and is coupled through condenser l09 to one end of a variable resistance H0 running to ground as shown. Atap I l I from the resistance 1 I 0 is then connected back to the second control grid of the tube T it being understood of course that this tube T is of the multigrid type,while the tubes T and T may be of any other desired type. In operation the circuit performs quite similar to that of Fig. 9, in that signals of ordinary intensity are amplified and fed through the circuit of tubes T and T without causing any action whatever of the trigger amplifier. When, however, the signal voltage appearing at the output of the tube T increases to the point that it will overcome the fixed bias on the trigger amplifier 'T ,this tube then functions to amplify this difference in signal voltage, and the'amplifiedcompo- 75 nent is fed to the second control grid of the amplifier T. This amplified signal, however, is out of phase with the input signal to the tube T andin effect cancels out a portion thereof in accordance with the amount of the bias overcoming signal voltage at the plate of the first amplifier tube. As a result a loud signal is automatically suppressed, and the output level of the tube T remains substantially constant at all times.

'In connection with the circuits of both Figs. 9 and 10, as Well as succeeding circuits embodying the phase inversion means, it may here be stated that the trigger amplifier tube is preferably of the variable [.t type, having .the remote plate current cut-off characteristics, since this type of tube has been found to function far better in this use.

Fig. 11 represents another ,of my possible circuit arrangements of the trigger amplifier or phase inverter wherein amplifier tubes T and 'I' are arranged in cascade to amplify a signal input from the device M and deliver the same to the load through the outputcondenser 44. In this instance the tubes T and T are indicated'as being of the triode variety with the usual plate load resistances and, grid biasing resistances. and 48, and interstage coupling condenser 49, there being also a direct current supply as indicated at for the plates of the tubes. In this case a portion of the input signal isalso fed to the control grid of the phase inverter tube T through the coupling condenser 5| and the output amplified signal from the tube T is connected through the coupling condenser 52 to bridging resistance 53 which effectively connects the outputof both tubes T and T The control grid of the second amplifier tube '1 is then adjustable across the resistance 53 to vary the relative signal peak up from the two tubes T and T The control grid of phase inverter tube T is provided with the biasing resistance 54 and also with a predetermined amount of fixed bias as represented at 55.

In'operation the signal ofweak or normal intensity is amplified by the; tubes T and 'I' in normal manner, but when a signal of greater intensity is encountered, it will overcome the bias on the tube T and will be'amplified thereby as well, this bias of course being adjustable in orde'r'to determine the level at'which this action takes "place. The signal'thus amplified by the tube T is applied through coupling condenser 52 and bridging resistance 53 to the input circuit of tube T but in a phase opposite to that of the signal' component .passed' by the tube T and the overall amplification. is thus automatically and instantaneously reduced to hold the signal output'in substantially a constant level. By proper phase inversion, to provide an amplifier having all of the desirable characteristics hereinbefore mentioned. In this circuit the tubes T andT represent the successive -or cascaded amplifier tubes'for amplifying the signal from the signal input device M and transmitting the same to the load through the output coupling device 51. The tubes T and 'I' are of the multi-g-rid type and are provided with the usual grid biasing resistances 58 and 59 as well as plate load resistances 60 and BI by which a direct current potential is received from the plate voltage supply 62. The screen grid 63 of the first tube is connected through the resistance 64 to the plate of the tube to provide the desirable degenerating action heretofore described and a connection is also made from the resistance 64 back to a portion of the succeeding tubes output circuit to provide the necessary signal controlled regenerating voltage. As here shown, this voltage is received from a portion of the plate load resistance BI. The circuit as thus far described, accordingly has the transconductance control and regenerating and degenerating action found to be so desirable. condenser 65 is employed and with the particular connection shown, the screen grid 66 of the tube T may be connected directly to the plate.

The phase inverter tube T4 has its control grid connected to the coupling condenser 61 to the output of the first amplifier tube T and this control grid is provided with adjustable biasing resistance 68 and fixed or initial bias 69. The plate of the tube T is connected to the plate load supply 62 through a load resistance .10 and is also connected to the coupling condenser II to a bridging resistance I2 from which the control grid of the second amplifier tube receives its signal input voltage in a manner substantially the same as that described with reference to Fig. 7.

In operation the amplifier circuit, due tothe actions hereinbefore set out will have a sub-. stantially uniform output as has been described. However, the relative proportion of the signal voltage fed back to the screen grid and plate of the first amplifier tube must be initially so adjusted that a sufficient regeneration will be obtained in order to provide maximum weak signal response. Under such circumstances it is possible that an extremely loud signal may not be suificiently suppressed by the automatic volume control action and as a result will provide peaks in the output or overloading of the amplifier. Such peak signals will in the present circuit be fed in part at least through coupling condenser 61 to the control gridof the phase inverter tube T and being sufiicient to overcome the initial bias on that tube, will be amplified and returned to the control grid circuit of the second amplifier tube T This signal voltage is out of phase with that appearing in the plate of thefirst tube and an automatic and instantaneous suppression of the loud signal or excess signal voltage will thus be secured. It will be evident, therefore, that by the combination of the three principlesof regeneration, degeneration, and phase inversion as provided in my invention, there may be obtained a substantially'uniform loud signal suppression without a weak signal suppression, thus maintaining substantially uniform output. In this circuit too the weak signal expanding regeneration may be increased beyond the point at which the circuit of Figjl mu operate, since the trigger tube will automatically reduce the regeneration as it approaches the point of fringe The usual interstage coupling howl. The operating level of the'first stage of D the ampifier circuit of Fig, l2'is, of course, more or less predetermined by the initial selection of the valves of the circuit components. The level of the amplifier as a whole, however, may be adjusted manually at any time by resistance 68 in the trigger amplifier circuit and this is first tube T accomplished without disturbance of the ratio of regeneration and degeneration by which weak signal expansion is obtained.

, Fig. 13 shows still another version of the trigger amplifier volume suppression circuit, in this case arranged between two cascaded amplifier tubes T and T only the input and output portions of which are here shown. The plate of the tube T is connected through the primary II2 of a transformer II3 to the B connection I I2a, and the center tapped secondary H4 is connected to a rectifier T This rectifier tube T -is provided with a fixed bias as represented at H5, and the center tapof the secondary H4 is connected through a resistance IIG to ground to provide an additional bias. A variable tap I II from this resistance I I6 is connected through coupling condenser II8 to the second control grid of the tube T and bias for this grid is provided from resistance H9. The said tube T is of the 'multi-grid type, of course, and has its input grid connected through resistance I20 and coupling condenser I2I to the output of the In operation the current variation set up in the plate circuit of the tube T by the amplified signal induces a current in the secondary of the transformer I I3, and when this current, under the influence of an extremely loud signal, exceeds the value of the fixed bias of the tube T rectification takes place and a voltage is present on the second grid of the tube T. This voltage, however, is out of phase with the incoming signal voltage resulting in partial suppression thereof in direct proportion to the amount to which the signal exceeds the predetermined value; and the desirable effect of loud signal suppression thus automatically and instantaneously takes place.

Although a single tube is indicated at T it is' obvious that two separate tubes may be employed, to provide the necessary .full wave or push-pull action.

In connection with both the circuits of Fig. 10 and Fig. 13, where in both the control grids of the second. amplifier tubes '1' are provided with variable input resistan'cesit' will be noted that the relative proportion of the in and out of phase signals fed to the grids may thus be readily controlled. It is thus possible to make volume control adjustments in the normal signal range and to thenlproperly adjust the out of phase signal input to compensate for such adjustments. In all of the circuits heretofore described it is of course understood that the feed back circuits and characteristics are always so arranged that the phase shift will never reach the point at which oscillation is caused. Since oscillation takes effect more readily at high frequencies than at low frequencies, the feed back or degeneration can be designed to attenuate the higher frequencies in exact proportion to their tendency to induce oscillation. In this manner and by proper design infinite gain may be obtained across the entire band of audio frequencies. Also then, and

1 following the same reasoning the feed back or generation may be replaced with choke 'coils,

coils tuned by capacitors or shunted by resistor or any other suitable coupling means.

It is to be understood, of course,'that whereever feasible the resistive plate loads in the trigger type degenerating circuits may be impedances such as choke coils, or choke coils tuned by capacitors or shunted by resistors, provided, however, that the circuits effected by the degenerative action are so designed as to have the same frequency characteristics as the impedances employed. That is, that the trigger network feeds back the same frequency characteristics whether the feed back gain is the same or difierent. Otherwise a certain band of frequencies may have no attenuation at all, while another band may have a maximum of attenuation. On the other hand, this may be desirable under such circumstances as when used with several separate amplifiers for speech scrambling as it is called.

As further explanatory of the trigger types of degenerative circuits (Figs. 10, ll, 12, and 13), it may be stated that, if the gain of the trigger tube T is greater than the gain of the amplifier tube T Whose circuit it effectively shunts when the initial bias is overcome, then the net gain to the following amplifier stage will be the gain of T multiplied by the gain of T minus the gain of T There thus results the effects of suppression, cancellation and expansion in one tube with the frequency characteristics controlled solely by initial design of the circuit components.

Although the invention is herein shown and described as embodied in circuits handling audio or speech frequencies, it is to be understood that my invention may be applied as well to circuits handling radio or intermediate frequencies, and such uses are considered within the scope of my invention. Having now therefore fully illustrated and described my invention, what I claim to be new and desire to protect by Letters Patent first stage employsa multi-grid tube, a resistance connected directly between a screen grid and the plate of said multi-grid tube, a direct current potential source, a plate load impedance connecting said source to the plate, and means for feeding back a signal voltage from the output of a following amplifying stage to the resistance between the screen grid and plate of said multigrid tube.

3. In a vacuum tube amplifier wherein a plurality of successive amplifying stages are employed and with multi-grid tubes in at least two early stages, a resistance connected between a screen grid and the plate of each of said multigrid tubes, an impedance plate load device in the plate circuit of each of said tubes, whereby a signal induced voltage drop in the said resistances and load devices will result in instantaneous variation in the gain of the tubes, and means for feeding back a signal voltage from one stage to the first so connected multi-grid tube for adding regeneration at weak signal inputs, and the said regeneration being controlled by the degenerative effect of said voltage drop in the said resistances and plate load devices.

4. In a signal amplifier system having a plurality of amplifying stages and at least one early stage employing a screen grid tube, a resistive connection directly between the screeen grid and plate of said tube, a load resistor connected to one end of said resistive connection and arranged to supply an operating potential to both screen grid and plate, and a feedback connection from a later stage of the amplifier to said resistive con nection and operative to feed back a regenerative signal voltage to the screen grid of the tube of the screen grid of the tube in said early stage.

5. In a signal amplifier system having a plurality of amplifying stages and at least one early stage employing a screen grid tube, a resistance connected directly between the screen grid and plate of said tube, a plate load resistor connected to one end of said resistance and operative to supply an operating potential to both screen grid and plate, a feedback connection from a later stage of the amplifier connected to said resistance and operative to feed back a regenerative signal voltage to the screen grid of the tube in said early stage, and means in said feedback "connection for controlling the frequency characteristics of the signal voltage feed back to the early stage.

FRANK J. FALTICO. 

